Update contribution guidance

Author: laike9m

docs/Contribute.md

@@ -30,6 +30,8 @@
 
    `solution.py` contains the right solution, with everything else unchanged.
 
+   **Rule of Thumb: ** A challenge should NEVER ask users to write the actual implementation. Users are expected to write type hints and type hints only, and be able to pass the tests.
+
 5. Test with [`pyright`](https://microsoft.github.io/pyright/#/installation?id=command-line) to make sure your new challenge works as expected.
 
 6. Create a Pull Request.